1. Field of the Invention
This invention relates to cast heat exchangers for hot water boilers and to a method of making such heat exchangers.
2. Discussion of the Prior Art
Hot water boilers, such as used in domestic central heating systems, employ a heat exchanger to transfer the heat from the burning fuel to water. Although heat exchangers based upon sophisticated materials and designs have been proposed and sometimes employed, the use of conventional cast heat exchangers with flueways and waterways is still prevalent, and indeed is preferred in many parts of the heating industry.
Such cast heat exchangers are normally (in the U.K.) formed of cast iron and, as such, are relatively heavy objects. An increasing trend in the market (especially for domestic systems) is for small, wall-mounted boilers and it goes without saying that there is a need for an efficient, relatively lightweight cast heat exchanger.
Efficiency of heat exchange and weight of cast metal are, to an extent, factors which contradict one another. Efficiency of heat exchange dictates a large surface area of cast metal in contact with hot fuel, yet any increase in this surface area can be locked upon as tending to increase the amount of cast metal required, and thus increasing the weight of the heat exchanger.
A typical wall-mountable domestic hot water boiler is the Potterton "Netaheat" (Trade Mark). This has a cast iron heat exchanger approximately 25 cm high, 18 cm wide (measured perpendicular to the wall-mounting surface), and 32 cm long (measured parallel to the wall-mounting surface). It has three connecting waterways and two flueways interposed between the waterways. This heat exchanger has a capacity of 13,200 to 16,100 W heat input into water with a non-fan assisted flue and this approximates to a heat input/weight of heat exchanger ratio of 340 W:kg of cast iron. It is to be observed that this heat exchanger is generally rectangular when viewed in plan (i.e. down onto the flueways) and the flueways run parallel to the longest axis: this arrangement is customary in the industry.
The "Netaheat" heat exchanger, in common with probably all cast heat exchangers, has two distinguishable types of heat exchange surface. The surfaces which may be said to form the walls of the flueways and contact most directly with the waterways can be termed "primary heat-exchange surfaces", whereas the surfaces which extend into the flueways from the primary heat-exchange surfaces (such surfaces being fins or the like) can be termed "secondary heat-exchange surfaces". The "Netaheat" heat exchanger has approximately 400 sq. in. (2,600 sq. cm.) of primary heat-exchange surface.
To manufacture heat exchangers such as the one described, the waterway sections can be cast separately in moulds and then subsequently assembled to provide the heat exchanger, or the sections can be cast together in one mould to provide an integrally-formed heat exchanger. The technique currently employed is to cast the waterways horizontally--that is to say, the two opposing walls of largest surface area are cast and formed horizontally, one above the other. As the molten metal is poured into the mould first one, then the next of these walls is formed. If a single waterway section is being formed obviously only two such walls exist and are formed, but if a heat exchanger comprising a plurality of waterways is being formed integrally, then each such waterway wall forms in the mould successively as the mould fills with molten metal. At any one time during moulding unequal pressure is exerted on the cores forming the waterways and experience has shown that these cores can bow under pressure of the metal. For this reason it has been customary to produce thicknesses for the majority of the walls in the heat exchanger no less than about 5 or 6 mm (for cast iron).
With the ever-rising cost of fuel for central heating purposes, the industry is facing demands for higher efficiency standards. To supply this there is a need to provide heat exchangers having closer tolerances in design criteria than in the past. The cost of the raw materials such as the iron for casting is also a factor to be taken into account. There is thus a need for cast metal heat exchangers using less metal than previously and which, at the same time, can be formed to the closer tolerances required thus enabling higher efficiency boilers to be produced.